Tare supporting apparatus

Abstract: A tare supporting apparatus supports the tare of a structure required to have very high surface precision, without affecting the surface precision of the structure. For that, the tare supporting apparatus comprises a driving unit for moving a driven body in the direction of gravity with respect to a base, and a supporting unit for supporting the tare of the driven body at a position after driven, wherein the driving unit and supporting unit are so arranged that a drive force of the driving unit and a support force of the supporting unit are on a same axis.

Description: The present invention relates to a supporting apparatus for supporting the weight of a machine structure, which may be applied to XY stages for semiconductor fabricating apparatus, precision machine tools, accurate measuring instruments, etc. Fig. 12 shows an example in which a supporting apparatus is mounted in a machine structure. In Fig. 12, numeral 1 designates a tilt stage. For example, for mounting a wafer 3, the top surface 2 of tilt stage 1 needs to have very high surface precision. Numeral 4 denotes a guide for restricting only a horizontal (Y-directional) motion of the tilt stage 1, which allows motions in the vertical direction (Z-direction), in an oblique direction, and in a direction of rotation about the Z-axis, for example, using a hydrostatic bearing. Through non-contact drive by a plurality of linear motors 26 (26a, 26b,...), a position along the Z-direction or the direction of gravity, or inclination of tilt stage 1 can be automatically controlled at a high speed with reference to a base 6. Also, numeral 27 designates an air cylinder, and one or more air cylinders support the tilt stage 1. Since the linear motors 26 transmit a force in a non-contact state, they have such an effective characteristic that Z-directional vibration of base 6 is not transmitted to the tilt stage 1. On the other hand, because a heat value of linear motor is proportional to the square of an electric current flowing therethrough, heat becomes enormous with an increase of drive force. Thermal deformation of machine structure caused by the heat would result in a fatal defect such as thermal deformation, for example, of a wafer mounted on the tilt stage as an accurate instrument. Accordingly, a design to minimize the drive force is required for applications of linear motor to accurate instruments. Thus, as in the above example, it is general that an air cylinder or the like is interposed so as to support the tilt stage. However, the above example for driving, for example, tilt of wafer, has the following problems. (1) The tilt stage could be deformed by excessive constraint because a point of drive by linear motors is not coincident with a support point by the air cylinder. (2) In order to keep an amount of deformation below a permissible value, it is necessary to receive the tare by a plane instead of the support point or to increase the thickness of tilt stage so as to enhance the rigidity of structure, which increases the size of apparatus. In either case, an amount of deformation is very small, but it would be a great error factor for structures requiring very high surface precision, such as the tilt stage for wafer. Thus, substantial improvement has been desired. The present invention has been accomplished in view of the above problems, and an object of the invention is to provide a supporting apparatus which can be used in structures requiring very high surface precision without affecting the surface precision. The present invention provides an apparatus for supporting a weight of a driven object with respect to a base, the apparatus comprising: driving means for generating a driving power for moving said driven object against gravity in a substantially vertical direction with respect to said base; and supporting means for generating a supporting power for supporting said weight of said driven object with respect to said base; characterised in that: said driving means and said supporting means are arranged to generate respectively said driving power and said supporting power on substantially the same axis, and a plurality of sets of said driving means and said supporting means are provided in said apparatus, whereby said driven object can be moved against gravity in said substantially vertical direction or inclined with respect to said base. In the present invention, the driven body can be held by a hydrostatic fluid bearing such as a porous hydrostatic bearing so as to restrict the degree of horizontal freedom relative to the base but to allow the other degrees of moving freedom relative to the base. The degree of horizontal freedom of the driven body is restricted by a fixed guide having a cylindrical guide surface located on the base, for example, and the driven body is supported through a fluid lubricating film in the radial direction of the fixed guide. The driving means is, for example, a linear motor, and the supporting means is, for example, an air cylinder. In this case, the linear motor is composed of a permanent magnet, a coil, and a member for holding the coil, and is preferably arranged to have a pipe line for flow of refrigerant in at least either one of the member for holding the coil or the surface of the coil. The air cylinder is composed of a cylinder, a piston, and a seal such as a bellow diaphragm. Each of horizontal cross sections of the cylinder, the piston, and the seal preferably has the shape composed of a pair of semi-circular arches and a pair of parallel straight lines connecting the semi-circular arches. Further, the apparatus is preferably provided with a control device for controlling the pressure in the air cylinder. The control device may use the pressure in the air cylinder or the electric current through the linear motor as a feedback signal. Since the above problems were caused by the fact that the drive point of linear motor and the support point are not coincident with each other, the present invention overcomes the problems by making them coincident with each other. Specifically, the linear motor and air cylinder are arranged on a same axis. Further, in order to facilitate assembling of apparatus and to effectively utilize the space, the air cylinder is arranged to have a cross section of track shape and is designed in a unit with the linear motor. Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a cross-sectional view of a tilt stage in which a tare supporting apparatus according to an embodiment of the present invention is mounted; Fig. 2 is an enlarged view of the tare supporting apparatus in Fig. 1; Fig. 3 is a cross-sectional view along AA' in Fig. 2; Fig. 4 is a cross-sectional view along BB' in Fig. 3; Figs. 5A and 5B are drawings to illustrate strain on a seal in the case of a rectangular cross section of piston; Figs. 6A and 6B are drawings to illustrate strain on a seal in the case of a circular cross section of piston; Fig. 7 is a drawing to show structure of an example of a drive system in the tare supporting apparatus of the present invention; Figs. 8A to 8D are drawings to show operational characteristics of the tare supporting apparatus of the present invention when a tilt stage is driven in the Z-direction; Fig. 9 is a drawing to show structure of a tare supporting apparatus according to the second embodiment of the present invention; Fig. 10 is a drawing to show a second example of the drive system in the tare supporting apparatus of the present invention; Fig. 11 is a drawing to show a third example of the drive system in the tare supporting apparatus of the present invention; and Fig. 12 is a cross-sectional view of a tilt stage in which a tare supporting apparatus is mounted. Fig. 1 shows a tilt stage in which supporting apparatus of an embodiment of the present invention is mounted. In Fig. 1, reference numeral 1 designates a tilt stage, and the top surface 2 of the tilt stage is required to have very high surface precision for mounting the wafer 3 thereon. Numeral 4 denotes a guide for restricting only movement of the tilt stage substantially in the Y- (horizontal) direction. For example, using a porous hydrostatic bearing, motions substantially in the Z- (vertical) direction, in an oblique direction, and in the direction of rotation about the Z-axis are allowed. Numeral 6 denotes a base. The guide 4 is fixed on the base 6 and a guide surface of guide 4 is cylindrical as extending in the Z-direction. Numeral 5 denotes the supporting apparatus of the present invention. Driving the three supporting apparatus (one of which is not shown), a position in the Z-direction or the direction of gravity or inclination of stage 1 can be automatically controlled at a high speed with reference to the base 6. Fig. 2 is an enlarged view of the supporting apparatus 5a in Fig. 1. Fig. 3 is a cross-sectional view along AA' of the supporting apparatus in Fig. 2. Fig. 4 is a cross-sectional view along BB' of the supporting apparatus in Fig. 3. The supporting apparatus shown in Figs. 2 to 4 is composed of a linear motor portion and a supporting portion. The linear motor portion is composed of permanent magnets 8, a coil 9, a coil holder 10 of non-magnetic body, a yoke 7 of magnetic body, and a yoke holder 11. The coil holder 10 is equipped with a cooling pipe 12 covering the coil in order to recover heat generated by the coil whereby a refrigerant can be made to flow from the outside through the pipe. The direction of a magnetic field of the permanent magnets and the direction of an electric current flowing through the coil are so arranged that the Lorentz force acts in the Z direction. In such an arrangement, the external size of the linear motor portion is determined so that the X-directional length is longer than the Y-directional length. The supporting portion is composed of a track-shape cylinder 13, a track-shape piston 15, and a track-shape bellow diaphragm seal 14. An air room 16 between the track-shape cylinder 13 and the track-shape piston 15 is perfectly sealed by the track-shape bellow diaphragm 14. The track-shape piston 15 is fixed to the coil holder 10. The track-shape cylinder 13 is fixed to the yoke holder 11. By this arrangement, a force generated by the linear motor and a force generated by the track-shape cylinder 13 work on a same axis along the Z direction, so as to drive the tilt stage 1 through the yoke holder 11. Each of horizontal cross sections of the track-shape cylinder 13, the track-shape piston 15, and the track-shape bellow diaphragm 14 has a shape, as shown in Fig. 4, composed of a pair of semi-circular arches and a pair of parallel straight lines connecting them. Although cross sections of normal bellow diaphragms are circular, the cross section of track shape is preferable to effectively utilize the rectangular space provided between the coil holder and the yoke in the linear motor portion. If the cross section of the bellow diaphragm were rectangular, enormous strain would occur at the corners of the bellow diaphragm in operation of cylinder, which would cause breakage of bellow diaphragm. This will be described with Figs. 5A, 5B and Figs. 6A, 6B. Figs. 5A and 5B show a bellow diaphragm having a rectangular cross section of piston and a small curvature for the corners, while Figs. 6A and 6B a bellow diaphragm having a circular cross section. In Figs. 5B and 6B (side views), when the cylinder moves ΔZ (from the solid line to the dashed line) in the Z direction, the bellow diaphragm expands ΔR from the chain line to the position of cylinder in Figs. 5A and 6A (top plan views). Then the strain ε of bellow diaphragm is expressed by the below formulas, where h is a gap between the piston and the cylinder and R is the diameter of piston. For the case of Figs. 5A and 5B, the strain at the corners is ε = {2πh - 2π(h/2)}/{2π(h/2)} = 1. For the case of Figs. 6A and 6B, ε = {h/2}/{R+h/2}«1. Since the track-shape bellow diaphragm has the shape composed of a pair of semi-circular arches and a pair of parallel straight lines connecting them, it can be approximated to the case of Figs. 6A and 6B, thus having small strain. The drive system in the supporting apparatus is next described. In Fig. 7, numeral 19 designates an air supply source. Numeral 17 is an air supply port and 18 an air exhaust port. Numeral 20 is a servo valve for controlling the pressure of air. Numeral 21 denotes a diaphragm, which is open to the air on the secondary side. Numeral 24 is a pressure controller for giving the servo valve an instruction. Numeral 22 is a pressure gauge for measuring the cylinder pressure. The pressure controller uses a signal 23 from the pressure gauge as a feedback signal, thus realizing a control system for keeping the pressure in the cylinder constant. Because of the constant pressure control system, the supporting portion has no static rigidity. Numeral 25 is a linear motor controller including a current amplifier, for driving the linear motor. Figs. 8A to 8D show operational characteristics of three types of supporting apparatus including the supporting apparatus of the present invention. Fig. 8A shows a time response of Z-directional displacement of the tilt stage when these supporting apparatus are driven. The response consists of a state before driving, a transient state, and a state after driving, and the transient state can be further separated into an acceleration period, a constant-speed period, and a deceleration period. Fig. 8B to Fig. 8D show time signals of current values flowing through the linear motors in the respective supporting apparatus at each moment. A time signal can be considered as equivalent to a drive force of linear motor. Fig. 8B shows a signal in a case without such a supporting portion as the air cylinder in the present invention. Fig. 8C shows a signal in a case employing a supporting method using a member with static rigidity, for example, such as a coil spring. Fig. 8D shows a signal in a case where the supporting apparatus of the present invention using the air cylinder is mounted as the supporting portion. In Fig. 8B, an offset appears before and after driving. This is because the linear motor always needs to produce a force balanced with the tare of the tilt stage, which is thermally disadvantageous as described previously. In Fig. 8C, no offset appears before driving but an offset appears after driving. This is because the linear motor produces no force before driving, i.e., at a balanced position between the tare and the coil, but the linear motor always needs to produce a force determined by a spring constant of the coil spring and a Z-directional displacement after driving. Although the spring constant of coil spring is generally small, such an arrangement is also thermally disadvantageous in the cases where the drive displacement is large, similarly as in the case of Fig. 8B. In Fig. 8D, there appears no offset of current value before and after driving, because the linear motor generates no static force in those periods. On the contrary, a large current flows in the transient state during driving, as compared with Fig. 8B. This is because the air cylinder used for supporting the tare serves as a spring element due to the compressibility of air against a momentary displacement. In order to decrease the transient current value, it is necessary to enhance the response of the pressure control system by the servo valve 20. The thermal effect, however, is very small, because the time period in which the current flows is short. The present invention is by no means limited to the above-described embodiment, but may have various modifications with necessity. For example, the following modifications are possible. (1) The above embodiment employs the track-shape air cylinder as the supporting portion, because there is a rectangular space between the coil holder and the yoke in the linear motor portion. Where a cylindrical linear motor such as a voice coil is employed, as shown in Fig. 9, a bellow diaphragm cylinder having a circular cross section can be used as the supporting portion. In this case, numeral 28 designates a cylindrical coil, 29 a cylindrical permanent magnet, 30a, 30b cylindrical yokes, and 31 a cylindrical coil holder. The coil holder 31 is equipped with a cooling pipe 12. Numeral 32 denotes a housing, and the housing and cylindrical yokes 30a, 30b constitute a cylinder. Numeral 33 is a cylindrical piston, 34 a cylindrical bellow diaphragm, and 35 an air supply port. (2) The above embodiment employs the track-shape air cylinder in order to be set in the rectangular space, but the air cylinder can be arranged with one or plural bellow diaphragm cylinders having a circular cross section. Since the pressure-receiving area in the cylinder decreases in that case, the pressure in the cylinder needs to be increased for supporting the tare. (3) The above embodiment employs the bellow diaphragm for seal. This is because it has advantages that no friction exists and that a perfect seal can be realized. For example, an 0-ring or a labyrinth seal can be used depending upon an object to be used. (4) The above embodiment employs the pressure control system which is a control system for keeping the pressure in the cylinder constant, but the control system can be constructed to drive the servo valve in such a manner that the electric current through the linear motor becomes zero, as shown in Fig. 10, using a value 36 of the current flowing through the linear motor as a feedback signal. In this case, the control can be realized with excellent response and accuracy. The control system can also be realized in such an arrangement, as shown in Fig. 11, that a mechanical diaphragm 37, for example, such as a regulator, is provided on either side of the air supply port and the air exhaust port without using the servo valve. In that case, stability of the regulator needs to be sufficiently high. According to the present invention, the tare can be supported without deformation of structure. Also, a decrease of load on the linear motors driving the structure and the flow of refrigerant can reduce an amount of heat emitted into the atmosphere from the linear motors. This results in decreasing a change in temperature of the structure and the atmosphere due to heating, which can suppress thermal deformation of structure and fluctuation of air, thus realizing high accuracy and improvement in reliability of apparatus. Also, because of the design of unit arrangement, assembling of apparatus is easy.

Claims: An apparatus for supporting a weight of a driven object (1) with respect to a base (6), the apparatus comprising: driving means (5) for generating a driving power for moving said driven object (1) against gravity in a substantially vertical direction with respect to said base (6); and supporting means (14, 15, 16) for generating a supporting power for supporting said weight of said driven object (1) with respect to said base (6); characterised in that: said driving means (5) and said supporting means (6) are arranged to generate respectively said driving power and said supporting power on substantially the same axis, and a plurality of sets of said driving means (5) and said supporting means (14, 15, 16) are provided in said apparatus, whereby said driven object (1) can be moved against gravity in said substantially vertical direction or inclined with respect to said base (6). An apparatus according to claim 1, further comprising a hydrostatic fluid bearing (4) for limiting a degree of freedom of said driven object (1) with respect to said base (6) in a horizontal direction and guiding said driven object (1) so that said driven object (1) can be moved with respect to said base (6) in a direction except for said horizontal direction. An apparatus according to claim 2, wherein said hydrostatic fluid bearing (4) includes a porous hydrostatic bearing. An apparatus according to claim 2, further comprising a fixed guide (4) having a cylindrical guiding surface disposed on said base (6), wherein said driven object (1) is supported through a fluid lubricating film in a radial direction of said fixed guide (4). An apparatus according to claim 1, wherein said driving means (5) comprises a linear motor and said supporting means comprises an air cylinder. An apparatus according to claim 5, wherein said linear motor includes a permanent magnet (8), a coil (9), and a member (10) for holding said coil (9), and at least one of said coil holding member (10) and said coil (9) is provided with a pipe line (12) for flow of refrigerant. An apparatus according to claim 5, wherein said air cylinder includes a cylinder (7), a piston (15) and a seal (14). An apparatus according to claim 7, wherein each of horizontal cross sections of said cylinder (7), said piston (15), and said seal (14) has a shape having a pair of semicircular arches and a pair of parallel straight lines connecting said semicircular arches. An apparatus according to claim 5, further comprising adjusting means (24) for adjusting a pressure within said air cylinder. An apparatus according to claim 9, wherein said adjusting means (24) is adapted to use said pressure within said air cylinder as a feedback signal. An apparatus according to claim 9, wherein said adjusting means is adapted to use a current signal of said linear motor as a feedback signal. A semiconductor fabricating apparatus having a supporting apparatus as claimed in any one of claims 1-11.

Inventors: AKUTSU KOTARO, KAMATA SHIGETO, OSANAI EIJI, AKUTSU, KOTARO, KAMATA, SHIGETO, OSANAI, EIJI, AKUTSU, KOTARO, C/O CANON KABUSHIKI KAISHA, KAMATA, SHIGETO, C/O CANON KABUSHIKI KAISHA, OSANAI, EIJI, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1994
Main classification: F16C32
IPC codes: B23Q1, H01L21, G03F7, F16C29, F16C32